1. Field of the Invention
The purpose of this invention is an iterative rake receiver and a corresponding reception process. It is used in radiocommunications applications and more particularly in the Code Division Multiple Access (CDMA) technique. It may be applied to the system defined by the IS""95 standard and in third generation UMTS and IMT-2000 systems.
2. Discussion of the Background
In the CDMA technique, information symbols that are to be transmitted do not directly modulate a carrier, but are firstly multiplied by pseudo-random sequences (or codes) which have the effect of spreading their spectrum. On reception, the received signal is unspread by filtering adapted to the sequence used in transmission (or by correlation) and is then demodulated.
This technique enables several users to use the same radiocommunications channel, provided that a particular sequence is assigned to each.
A multipath channel is generally used, in the sense that the radio-electric wave propagates along several different paths between the place of transmission and the place of reception. Therefore, the receiver does not receive a single signal for each transmitted information symbol, but it receives several more or less delayed and more or less modified copies. In order to reliably restore the transmitted information, the largest possible number of these copies has to be taken into account and they have to be recomposed in the receiver.
One way of doing this was to design a special receiver, called a rake receiver, in the sense that it xe2x80x9crakesxe2x80x9d information at different instants symbolizing the teeth of the rake. This type of receiver separates signals corresponding to different paths and includes several xe2x80x9cteethxe2x80x9d or xe2x80x9cbranchesxe2x80x9d or xe2x80x9cpinsxe2x80x9d, each of which processes one of these signals. The signal is unspread and demodulated in each tooth. The next step is to recompose all signals in an adder.
A rake receiver was initially described by R. PRICE and P. E. GREEN in an article entitled xe2x80x9cA Communication Technique for Multipath Channelsxe2x80x9d published in the xe2x80x9cProceedings of the IRExe2x80x9d, vol 46, March 1958, pp 555-570.
There is also a description of this receiver in the general book by J. G. PROAKIS entitled xe2x80x9cDigital Communicationsxe2x80x9d, 3rd edition, MCGRAW-HILL, 1995, (third edition) 1989, (second edition).
FIG. 1 attached diagrammatically shows a receiver of this type. As shown, this receiver comprises a main input E, a filter 10 with a width adapted to the signal spreading band, L means 120, . . . , 12l, . . . , 12Lxe2x88x921 of restoring L signals unspread in frequency corresponding to L paths (these means usually include a filter adapted to one of the pseudo-random sequences used in transmission or a correlator and means of searching for signal peaks), L means 140, . . . , 14l, . . . , 14Lxe2x88x921 of estimating the characteristics of the L paths used by the various signals, L demodulation means 160, . . . , 16l, . . . , 16Lxe2x88x921 combining the unspread signals and estimate of paths, an adder 18 adding the N contributions output by the N demodulators, and finally a decision circuit 20 that outputs transmitted or reference symbols on a general output S in order to test the communication.
After a strong demand from services requiring ever increasing throughputs, the frequency spreading band of CDMA systems is continuously being increased. This increase in the band is accompanied by a continuous increase in the number of paths received at the receiver. For a given received power, this increase in the number of paths tends to reduce the power received per path and therefore to reduce the global quality of the channel estimate. Consequently, a constructive combination of contributions from these paths to the receiver is rarely guaranteed and can result in a severe loss of transmission quality.
Since CDMA systems are inherently limited by multiple access interference, this loss of performance cannot be compensated by increasing the power. Furthermore, a solution involving an increase in the number of pilot symbols would reduce the system capacity.
The purpose of this invention is to correct this disadvantage.
The main purpose of this invention is to increase the performances of CDMA systems by improving the reception quality for a given transmitted power, and therefore with an unchanged level of multiple access interference. This quality improvement can increase the capacity and coverage of the CDMA system. This improvement is obtained by optimizing operation of the receiver in the classical case of slow fading, but also in the more difficult case of very fast fading.
Another purpose of the invention is to facilitate the construction of terminals by making them much less sensitive to inaccuracies in the local oscillator used to transpose the received signal in the base band.
CDMA systems introduce the concept of a power control period (PCP). The power of the signal emitted by the transmitter remains constant during each of these periods but it can vary from one power control period to the next in order to compensate for slow fading (due to the distance and mask effects), and fast fading due to multipath effects when the terminal is moving slowly. Furthermore, control symbols are used in addition to symbols transporting information. The invention can reduce the relative number and/or power of these control symbols, for an equal reception quality. This objective is achieved by taking optimum account of control symbols for an arbitrary number of consecutive power control periods in the channel estimate. It is also achieved by taking account of control symbols (if any) included in the pilot channels of some CDMA systems in the optimal channel estimate, in the absence of matching antennas. It is also achieved by taking account of control symbols (if any) assigned to other users in the down link in the estimate, still in the absence of matching antennas. Finally, it is achieved by taking optimum account in the estimate of this channel of all or some of the data symbols for these power control periods which are obviously more numerous and often contain more energy than the control symbols. The reduction in the number and/or the power of the control symbols is a way of consolidating the encoding of useful data and increasing the proportion of the transmitted power allocated to these data.
With the invention, the optimum channel estimate can also take account of control symbols multiplexed in time and control symbols multiplexed on in-phase and/or in-quadrature components of the modulated signal.
All power control period symbols can be used with the invention. Thus, shifts in the local oscillator frequency can be monitored and corrected, even if control symbols are grouped together.
According to the invention, block by block processing is carried out every time that the received signal corresponding to a given number of power control periods is available. Like a conventional rake receiver, it always begins by unspreading the signals corresponding to significant paths selected for the final combination. It then makes an approximate estimate of the multipath channel by using control symbols associated with the received block only. This estimate is a way of characterizing the variation in the phase and the amplitude of each path during the block to be processed, for each symbol in the block, symbol by symbol. The receiver according to the invention demodulates and then combines the estimated path contributions and outputs a sample (or weighted output) for each data symbol contained in the block.
In the case of a conventional rake receiver, these weighted outputs are used directly to detect and decode transmitted data symbols. These outputs have a certain reliability with respect to values taken on by the data symbols sent during a block. In the case of the receiver according to the invention, they may be used in addition to the control symbols, to provide an improved estimate of.each received path. This improved estimate of the multipath channel may be optimized, possibly tanking account of the encoded structure of the data symbols. Taking account of the correcting encoding results in better quality of the weighted outputs at the receiver.
The weighted outputs obtained at the end of a given iteration may be used again, together with control symbols, to provide an additional improvement to the channel estimate. This improved estimate then improves the quality of the weighted outputs generated by the receiver. Therefore, the receiver output is looped back onto the estimating means.
The optimum nature of the receiver according to the invention is related to the nature of the estimate of the multipath channel. This optimum nature depends firstly on the use of an iterative Estimation-Maximization (EM for short) type algorithm to construct the most likely channel, as a function of the received block. For example, this algorithm is described in the article by A. P. DEMPSTER, N. M LAIRD and D. B RUBIN entitled xe2x80x9cMaximum Likelihood from Incomplete Data via the EM Algorithmxe2x80x9d, published in the Roy.Stat.Soc journal, Ser. 39, 1977.
The optimum estimate for the channel also depends on the decomposition of each path received according to an expansion algorithm called the KARHUNEN-LOEVE algorithm. This decomposition enables flexible characterization of time variations of paths due to the oppler effect and can easily be included in the EM lgorithm itself. For example, the KARHUNEN-LOEVE lgorithm is described in the book by J. G. PROAKIS entioned above, 1989 version, pages 340-344.
More precisely, the purpose of this invention is a CDMA radiocommunication signals receiver, these signals having been obtained from spectrum symbols spread using pseudo-random sequences, these signal then having been propagated along a number of paths, this receiver comprising:
means of restoring L unspread signals for each symbol, corresponding to L different paths,
means of calculating L estimates of the L paths,
demodulation means for processing each of the L unspread signals using the L corresponding estimates to obtain L path contributions,
an adder to form the sum of these L contributions and to output an estimate of the received symbol,
a decision circuit about the received symbol starting from the value of the estimate output by the adder,
this receiver being characterized in that:
a) it processes blocks of N symbols, each block comprising data symbols and control symbols, each symbol being identified by the rank k that it occupies in the block, where k varies from 0 to Nxe2x88x921,
b) for each path identified by an index l, where l varies from 0 to Lxe2x88x921, and for each block, the receiver considers a vector Cl with N components that characterizes the path during this block,
c) the receiver comprises means of defining a vector base Bk, these vectors being N eigenvectors of the matrix E[ClCl*T], each vector Cl being decomposed in this base, the decomposition coefficients denoted Glk forming independent random Gaussian variables,
d) coefficients Glk, defining a vector Gl with N components for each path l, the estimating means being capable of estimating each vector Gl, using an iterative process based on EM estimation-maximization algorithm based on a maximum a posteriori probability criterion.
In a particular embodiment, the output from the adder is looped back onto the estimating means, the estimating means being used initially making use of control symbols contained in the block and assumed to be known, so that a first estimate of the data symbols contained in the block can be obtained from the output of the adder, the estimating means then using all estimated symbols present at the output from the adder and so on, the estimating means finally outputting the optimum value of Gl (l=0, 1, . . . , Lxe2x88x921), after a final iteration.
Another purpose of this invention is a process for reception of CDMA type of radiocommunication signals, these signals having been obtained from spectrum symbols spread using pseudo-random sequences, these signals then having been propagated along a number of paths, this reception process comprising the following operations:
for each symbol, L unspread signals corresponding to L different paths are restored,
L estimates of the L paths are calculated,
each of the L unspread signals is demodulated using the L corresponding estimates to obtain the L contributions of the paths,
the sum of these L contributions is formed, which gives an estimate of the received symbol,
a decision is made about the received symbol, based on the value of the estimate obtained,
this process being characterized in that:
a) blocks of N symbols are processed, each block comprising data symbols and control symbols, each symbol being identified by rank k that it occupies within the block, where k varies from 0 to Nxe2x88x921,
b) for each path, identified by an index l where l varies from 0 to Lxe2x88x921, and for each block, a vector Cl with N components is considered, which characterizes the path during this block,
c) the matrix E [ClCl*T] which has N eigenvectors denoted Bk is considered, and these N eigenvectors Bk are used as a base, each vector Cl in this base is decomposed, the decomposition coefficients denoted as Glk forming independent Gaussian random variables,
d) the coefficients Glk defining a vector Gl with N components for each path l, and each vector Gl is estimated using an iterative process based on an estimation-maximization (EM) algorithm based on a maximum a posteriori probability criterion.
In one particular embodiment, the iterative process is used initially taking account of the control symbols contained in the block and assumed to be known, which gives a first estimate for data symbols contained in the block, the said iterative process then taking account of all symbols in the block according to this first estimate, which can give a second estimate of the symbols in the block, and so on, until a satisfactory estimate is obtained for values of Gl, that are used for the demodulation.